In last ten years in some caves of Croatian Dinaric karst of Croatia concentration of Radon gas was measured. Only in some measured caves on fissures or joints concentration were larger than on faults inside the caves. It is strange but it could be explained by intensive air circulation in places were active faults made cave channels. Fissures and joints are mainly very near narrow and closed in caves but radon gas still passing through this places in caves better that trough some active faults. The same phenomena were measured in some caves on Cyprus in March and October 2017 and April 2018. Air pressure, altitude, temperature, groundwater characteristics, position of entrance, air circulation in caves, lithostratigraphic factors of rocks, neotectonic activity etc were compared to get some answers about this relations. On Cyprus the deepest explored cave is about 205 meters deep and the longest about 375 meters long (January 2019). In Croatia the deepest cave is about 1431 meters deep and the longest cave is about 37389 meters long (January 2019).Totally on Cyprus are known about 220 caves and in Croatia is known about 12000 caves. International team of scientists from Cyprus, Lebanon, France, USA and Croatia explored and measured concentration of Radon gas on special places of geological discontinuity in some caves on Cyprus in year 2017 and 2018. The name of this EU project is: « The Caves of Kyrenia Mountain Project – Research, Conservation and Education ».